The traditional X-ray systems used for medical applications are mainly based on the different absorption of X-rays in matter resulting in projection images formed by the accumulated absorption of the X-rays along a line. Yet, matter does not only influence the absorption but also the refraction of X-rays. This characteristics promises higher X-ray contrast between human tissue types than the conventional absorption-based imaging and hence can represent a future improvement. A possible setup for facilitating PCI is based on gratings.
The use of gratings within the transmitted X-ray beam face the problem, that non-focused gratings produce a less modulated absorption by themselves especially outside of the central region of the field-of-view (FOV), resulting in a decreased visibility of an interference pattern, a decreased transmitted mean intensity, and thus a lower signal-to-noise ratio at a detector. Therefore, the absorption image has either lower contrast-to-noise ratio or the radiation dose must be increased which the human body is potentially harmfully exposed. For the same reason, a typically non-focused source grating limits the FOV which can be exploited effectively by the PCI system. This last aspect is caused by both the thickness of the source grating necessary for producing a minimal absorption for the X-rays and the slit width needed to filter out sufficiently coherent beams. All that leads to a decreased mean X-ray intensity and a decreased visibility of the interference pattern for locations in the resulting image outside the centre of the FOV.
A problem of differential phase contrast CT is that wrapping of the phase gradient occurs at the rim of the object. Different methods to unwrap the phase gradient have been proposed already, improving overall image quality substantially, but nevertheless, none of them works perfectly. Another approach is to avoid the phase wrapping by means of additional hardware. A further drawback of differential phase contrast CT is the fact that the noise power spectrum of the image has a peak for low frequencies. The problem here is that low-frequency noise is perceived more annoying than high frequency noise.